1. Field of the Invention
The present invention relates to an apparatus for forming a continuous elastomeric strip, such as in the nature of a tire tread slab, and more particularly to a method and apparatus in which there is utilized an assembly comprised of a plurality of extruders feeding different compositions of elastomeric material into a preform die that joins the different compositions to form a multi-material slab that is extruded through a roller die as a continuous elastomeric strip.
2. Description of Related Art
Generally speaking, conventional methods of producing tire tread slabs utilize apparatus that extrude elastomeric materials under pressure through slit-dies that are contoured to appropriately impart to the elastomeric material, of which the tread slab is constituted, a preferred cross-sectional contour.
In addition, there are also methods that utilize roller-die combinations that extrude elastomeric material under pressure between a restriction orifice and a roller die. This later type apparatus is disclosed in U.S. Pat. No. 3,871,810 assigned to UNIROYAL, Inc.
Both of these types of extrusion methods have been adapted in recent years to form elastomeric strips that are comprised of two or more different types of material or extrudate. This normally requires internal flowpaths that initially start as two or more separate flow channels originating with two or more separate extruders, eventually joining together to combine the separate materials within a single slab that ultimately is formed into a tire tread. The advantage of combining different types of elastomeric material into a single tire tread is that each material can be used in a specific physical section of the tread where the material has the most advantageous physical properties for improving overall performance of the tire tread. Specifically, it has been found that it is desirable to utilize one material in the radially outer center portion (cap portion) of the tread which is the part that normally contacts a road surface. This material might have a high coefficient of friction for improving vehicle handling characteristics. It might also be a highly wear-resistant material for the purpose of improving tire tread life.
In contrast, the lateral portions of the tire tread, also known as the buttress of the tire tread, would most desirably be made of a different material such as a highly elastic and flex resistant material that can absorb major stresses encountered during significant changes in direction of a subject vehicle. This compound must also be compatible with tire sidewalls to form a cured junction between the sidewall and tread which will not separate during its intended service. This buttress portion of the tread is also known as the "wings" of the tire tread.
In addition, that portion of the tread that underlies the wear resistant surface, sometimes called the cap base or tread recoat, might be formed from the same material as the "wings" or a third material designed to adhere to the outer stock of the tire carcass and supply a modulus transition between the tread portion and a top carcass skimcoat. If this third material has a low hysteresis, then proper control of the contour and volume of this material is desirable to improve the tire power consumption for vehicle fuel efficiency by reducing tire rolling resistance.
While these different materials will generally have markedly different viscosities, they must be brought together to form a single cohesive elastomeric strip. Normally this will be accomplished inside an extrusion head with multiple flow channels that, at some point, are joined together.
The general practice in the prior art has been to join multiple materials or compositions at a physical location just before the elastomeric strip is extruded out onto a moving belt or onto a rotating roller. It has been generally assumed that it is necessary to join the different materials at or near the final exit of a multi-material extrudate because it would be difficult or impossible to control the boundary between the different materials within the elastomeric strip. Additionally, there has not been any widely known advantage to joining the different materials at a point significantly upstream of the final exit of the extrudate.
Previous technology has also been based on an assumption that the direction of the flow channels must be changed gradually into the shape required for a final extruded profile because any rapid changes in direction would cause extrusion difficulties. Specifically, it has been believed that any major turns or changes in direction in the passages before the junction is formed would destabilize the final extrudate causing variations in boundary location. Also, it has been generally believed that the different materials must be advanced to their junction at the same velocity and pressure to the point where the junction is made.
These assumptions have lead to unnecessary equipment complications, difficulties in die design and restrictions in extrudate or tire tread design.
Associated with the problem of maintaining proper material boundary location is the desire on the part of tire manufacturers to be able to vary the boundary location, within a small range, without having to resort to changing dies within the extrusion equipment. This kind of control within a small range would permit fine tuning of equipment to more precisely control material boundary location during equipment operation, instead of being forced to shut machinery down for die substitution.
Another difficulty that is a constant concern to manufacturers of tire tread extrusion equipment is the problem of providing convenient access to the flowpaths within the equipment to enable operators to efficiently clean the equipment. This is a greater problem than might normally be expected because tire manufacturers frequently have to interrupt extrusion operations to change dies for different tire sizes or to accommodate shift changes or production holdups. Every time these interruptions occur, the elastomeric material already in the machine tends to harden inside the flowpaths and it must be cleaned out before the next run is started. This requires labor and, worse yet, shuts down expensive equipment longer than necessary.
To minimize these problems, extrusion equipment manufacturers are incorporating more and more features into the equipment to permit easier cleaning of the internal flowpaths. In spite of these efforts, equipment downtime during cleaning remains a major concern to extrusion equipment manufacturers.
Finally, it has been a constant concern of the tire tread equipment manufacturers to provide a method and apparatus that extrudes two or more materials in a strip with a high degree of adhesion between the different materials at the boundary location.